This invention relates to web tensioning devices for machines which apply a generally helical winding of a wrapping web by using a shuttle which contains a supply of the wrapping web and which moves around a track defining a closed path. The invention particularly relates to axial winding of a wrapping web in which the closed path of the track passes through an opening of an object to be wrapped.
In machines of the type described above, relative movement is established between the object to be wrapped and the closed path around which the shuttle moves. Such machines can be used to apply a wrapping to elongate articles or bundles of elongate articles and especially can be used for wrapping annular or toroidal products such as steel coils. In this case the track is either formed in two halves as described in U.S. Pat. No. 5,755,083 or includes a gate as shown in U.S. Pat. No. 5,282,347 to enable the coil to be positioned such that the track goes through the centre of the coil and around the outside of the coil. The track is usually arranged to be fixed in space and the coil arranged to be rotated about its axis so that, as the shuttle moves around the track, axial wrapping takes place so that a generally helical winding of packaging web is applied around the outside of the annular coil and through its centre aperture.
The rate of removal of the web from the shuttle varies as the shuttle moves around the track and depends upon the size of the object. Again taking the case of steel coils, these usually have a standard centre aperture but the width of the steel sheet and the length of the sheet (and therefore the diameter of the coil) often vary from coil to coil and thus the generally rectangular cross-section of the coil being wrapped varies from coil to coil. To accommodate such variations most shuttles include some form of brake mechanism to brake the unwinding of the web supply reel and an accumulator to hold a supply of web to allow it to be taken at an irregular rate. To enable such shuttles to fit through the centre of a steel coil the shuttle and track must be made as small as reasonable and it would be desirable to avoid an accumulator on the shuttle. Some attempts have been made to do this in, for example, U.S. Pat. No. 5,829,234 in which the shape of the track is varied to provide a substantially uniform rate of removal of the web from the shuttle and so avoid the need for an accumulator on the shuttle.
When the web is tensioned by using a simple reel brake then the tension in the web varies with the radius of the supply reel and therefore the tension in the wrapping varies as the web is dispensed from the supply reel. It has been proposed in EP-A-0936141 to provide a variable brake for the supply reel, a tension sensor for sensing the tension in the web downstream from the supply reel and, feedback means to control the supply reel brake in response to the sensed tension to maintain the tension substantially constant. Similar feedback arrangements are used on large scale fixed web feeding systems used with printing presses and film reeling machines, but it is difficult to use such techniques in a shuttle moving around a closed path. Typically, the power to drive a shuttle round such a closed path is taken from wiper contacts on the shuttle which engage conductor rails located around the track. The use of sophisticated control circuitry relying on such wiper contacts is difficult to implement in practice.
According to a first aspect of this invention a web tensioning device for a shuttle applying a generally helical winding includes a pair of substantially rigid idle rollers covered with resilient material and urged against one another to form a nip with their resilient covering compressed in the nip and with the web being fed between the nip of the rollers.
We have found that such a simple arrangement provides a substantially constant tension in the wrapping web that does not vary with the radius of the supply reel or vary significantly with the rate of unreeling of the wrapping web. This arrangement certainly provides results which are very much better than those of a simple supply reel brake and is much easier and cheaper to implement than the systems involving a variable braking effort on the supply reel.
Such web tensioning devices can be used with a machine as described in EP-A-0936141 where the shape of the track is varied to provide a substantially constant rate of unreeling of the wrapping web and thus not require an associated web accumulator. However, it is preferred that the shuttle also includes a web accumulator downstream of the web tensioning device to enable the shuttle to cope with changes in the instantaneous rate of web supply from it.
Preferably, the rollers have a hollow or solid steel core with a covering of rubber or rubber-like elastomeric material, the rubber or rubber-like elastomeric material may be a vulcanised natural or synthetic rubber or a thermoplastic elastomer such as polyurethane. Preferably, the resilient covering is heat resistant since substantial quantities of heat can build up in the rollers at high web throughput rates. Preferably, the covering has a Shore hardness of between 70 and 75 and a specific gravity of between 1.14 and 1.20. The core of the rollers can have a diameter from between 25 to 100 mm and a covering thickness in a range of 8 to 4 mm, respectively.
Preferably, the rollers are mounted in a box frame construction consisting of two side plates and stretchers which form the body of the shuttle, the journal bearings for one of the rollers being mounted on slides in the side plates and including adjusters such as jack-screws to urge the one roll towards the other. Preferably, a web supply roll on a web supply reel is urged against the up-stream roller or the up-stream roller is urged against the web supply roll to control the unwinding of the web from the roll and it does not allow the supply roll to freewheel.
In a second aspect, the invention consists in a shuttle for carrying a web material adapted to be applied in a generally helical winding, the shuttle including a web tensioning device according to the first aspect.
In a third aspect, the invention consists in wrapping apparatus for applying a wrapping web to an article, the wrapping apparatus including an endless track arranged about the article to be wrapped and upon which at least one shuttle according to the second aspect travels.